Porous metal oxide microspheres with varying pore sizes

ABSTRACT

Porous metal oxide microspheres are prepared via a process comprising forming a liquid solution or dispersion of polydisperse polymer nanoparticles and a metal oxide; forming liquid droplets from the solution or dispersion; drying the liquid droplets to provide polymer template microspheres comprising polymer nanospheres and metal oxide; and removing the polymer nanospheres from the template microspheres to provide the porous metal oxide microspheres. The porous microspheres exhibit saturated colors and are suitable as colorants for a variety of end-uses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 16/126,346, filed on Sep. 10, 2018, which claims the benefit of priority of U.S. Provisional patent Application No. 62/556,798, filed on Sep. 11, 2017, the disclosures of which are hereby incorporated by reference herein in their entireties.

BACKGROUND

Traditional pigments and dyes exhibit color via light absorption and reflection, relying on chemical structure. Structural colorants exhibit color via light interference effects, relying on physical structure as opposed to chemical structure. Structural colorants are found in nature, for instance in bird feathers, butterfly wings and certain gemstones. Structural colorants are materials containing microscopically structured surfaces small enough to interfere with visible light and produce color. Such materials may be based on photonic materials including, but not limited to, opals, inverse opals, photonic granules, photonic spheres or composite photonic crystals. The term “photonic material” refers to a material having a degree of periodic variations in its structure.

Structural colorants may exhibit high stability. Accordingly, desired are structural colorants that exhibit different colors of visible light observable to the naked eye when present in bulk. Such structural colorants may be formulated into consumer products as a replacement for less stable and/or less environmentally friendly pigments or dyes.

It has been found that certain porous metal oxide microspheres exhibit high quality color in bulk. The microspheres provide color visible in the bulk.

SUMMARY

Accordingly, disclosed is a method to prepare polymer microspheres comprising polydisperse polymer nanospheres, the method comprising forming a liquid solution or dispersion of monodisperse polymer nanoparticles; forming at least one further liquid solution or dispersion of monodisperse polymer nanoparticles; mixing each of the solutions or dispersions together; forming droplets of the mixture; and drying the droplets to provide polymer microspheres comprising polydisperse polymer nanospheres; wherein the average diameters of the monodisperse polymer nanoparticles of each of the solutions or dispersions are different.

Also disclosed are polymer microspheres comprising more than one population of monodisperse polymer nanospheres, wherein each population of monodisperse polymer nanospheres has different average diameters. A bulk sample of the polymer microspheres may exhibit color observable by the human eye.

Also disclosed is a method to prepare porous metal oxide microspheres, the method comprising forming a liquid solution or dispersion of monodisperse polymer nanoparticles; forming at least one further liquid solution or dispersion of monodisperse polymer nanoparticles; mixing each of the solutions or dispersions together; wherein a metal oxide is added to one or more of the solutions or suspensions and/or wherein a metal oxide is added to the mixture to form a liquid dispersion of polymer nanoparticles and metal oxide; forming liquid droplets of the liquid dispersion; drying the droplets to provide polymer template microspheres comprising polydisperse polymer nanospheres and metal oxide; and removing the polymer nanospheres from the template microspheres to provide the porous metal oxide microspheres; wherein the average diameters of the monodisperse polymer nanoparticles of each of the solutions or dispersions are different.

Also disclosed are porous microspheres comprising a metal oxide, wherein the microspheres have an average diameter of from about 0.5 μm to about 100 μm and an average porosity of from about 0.10 to about 0.90 or from about 0.10 to about 0.80; wherein the porous microspheres have more than one population of pores each population having an average pore diameter, wherein each population has a different average pore diameter; and wherein the average pore diameters are from about 50 nm to about 999 nm; for example, wherein the microspheres have a first population of pores having an average pore diameter of from about 50 nm to about 999 nm and a second population of pores having an average pore diameter of from about 50 nm to about 999 nm, wherein the first and second average pore diameters are different.

Disclosed also are porous microspheres comprising a metal oxide, wherein a bulk sample of the porous microspheres exhibits color observable by the human eye.

Disclosed also are compositions comprising a substrate and the present microspheres; for example where the compositions are aqueous formulations, oil-based formulations, coatings formulations, inks, foods, plastics, cosmetic formulations or materials for medical applications or security applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure described herein is illustrated by way of example and not by way of limitation in the accompanying figures. For simplicity and clarity of illustration, features illustrated in the figures are not necessarily drawn to scale. For example, the dimensions of some features may be exaggerated relative to other features for clarity. Further, where considered appropriate, reference labels have been repeated among the figures to indicate corresponding or analogous elements.

FIG. 1 shows a general outline for the preparation of present porous microspheres.

FIG. 2 is a scanning electron microscope (SEM) image of a polymer template microsphere, according to an embodiment of the invention.

FIG. 3 is a SEM image of a porous silica microsphere, according to an embodiment of the invention.

FIG. 4 is a representation of a spray-drying process according to some embodiments of the invention.

DETAILED DESCRIPTION

Present metal oxide microspheres, or photonic balls, may be prepared with the use of a polymeric sacrificial template. In one embodiment, an aqueous colloid dispersion containing polymer nanoparticles and a metal oxide is prepared, the polymer particles typically being nano-scaled. The aqueous colloidal dispersion is mixed with a continuous oil phase, for instance within a microfluidic device, to produce a water-in-oil emulsion. Emulsion aqueous droplets are prepared, collected and dried to form microspheres containing polymer nanoparticles and metal oxide. The polymer nanoparticles (nanospheres) are then removed, for instance via calcination, to provide spherical, micron-scaled metal oxide particles (microspheres) containing a high degree of porosity and nano-scaled pores. The microspheres may contain varied pore diameters, a result of the polymer particles being polydisperse.

FIG. 1 shows a general outline for the preparation of present porous microspheres. An emulsion droplet containing polymer nanospheres and metal oxide is dried to remove solvent, providing an assembled microsphere containing polymer nanospheres with metal oxide in the interstitial spaces between the polymer nanospheres (template microsphere or “direct structure”). The polymer nanospheres are polydisperse. The polymer nanospheres define the interstitial space. Calcination results in removal of the polymer, providing a present metal oxide microsphere with high porosity, or void volume (inverse structure).

The porous metal oxide microspheres are advantageously sintered, resulting in a continuous solid structure which is thermally and mechanically stable.

In some embodiments, droplet formation and collection occurs within a microfluidic device. Microfluidic devices are for instance narrow channel devices having a micron-scaled droplet junction adapted to produce uniform size droplets connected to a collection reservoir. Microfluidic devices for example contain a droplet junction having a channel width of from about 10 μm to about 100 μm. The devices are for instance made of polydimethylsiloxane (PDMS) and may be prepared for example via soft lithography. An emulsion may be prepared within the device via pumping an aqueous dispersed phase and oil continuous phase at specified rates to the device where mixing occurs to provide emulsion droplets. Alternatively, an oil-in-water emulsion may be employed.

In some embodiments, vibrating nozzle techniques may be employed. In these techniques, a liquid dispersion is prepared; droplets are formed; and the droplets are dropped into a bath of a continuous phase. The droplets are then dried and calcined. Vibrating nozzle equipment is available from Büchi and comprises for instance a syringe pump and a pulsation unit. Vibrating nozzle equipment may also comprise a pressure regulation valve.

The polymer nanoparticles for instance have an average diameter of from about 50 nm to about 999 nm. The polymer nanospheres are polydisperse. According to the present invention, a polydisperse sample of polymer nanospheres contains more than one monodisperse population of polymer nanospheres, that is, at least first and second monodisperse populations of polymer nanospheres, where the first and second average particle sizes are different.

Suitable template polymers include thermoplastic polymers. For example, template polymers are selected from the group consisting of poly(meth)acrylic acid, poly(meth)acrylates, polystyrenes, polyacrylamides, polyvinyl alcohol, polyvinyl acetate, polyesters, polyurethanes, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, polyvinyl ethers, derivatives thereof, salts thereof, copolymers thereof and combinations thereof. For example, the polymer is selected from the group consisting of polymethyl methacrylate, polyethyl methacrylate, poly(n-butyl methacrylate), polystyrene, poly(chloro-styrene), poly(alpha-methyl styrene), poly(N-methylolacrylamide), styrene/methyl methacrylate copolymer, polyalkylated acrylate, polyhydroxyl acrylate, polyamino acrylate, polycyanoacrylate, polyfluorinated acrylate, poly(N-methylolacrylamide), polyacrylic acid, polymethacrylic acid, methyl methacrylate/ethyl acrylate/acrylic acid copolymer, styrene/methyl methacrylate/acrylic acid copolymer, polyvinyl acetate, polyvinylpyrrolidone, polyvinylcaprolactone, polyvinylcaprolactam, derivatives thereof, salts thereof, and combinations thereof.

In certain embodiments, polymer templates include polystyrenes, including polystyrene and polystyrene copolymers. Polystyrene copolymers include copolymers with water-soluble monomers, for example polystyrene/acrylic acid, polystyrene/poly(ethylene glycol) methacrylate, and polystyrene/styrene sulfonate.

Present metal oxides include oxides of transition metals, metalloids and rare earths, for example silica, titania, alumina, zirconia, ceria, iron oxides, zinc oxide, indium oxide, tin oxide, chromium oxide, mixed metal oxides, combinations thereof, and the like.

The wt/wt (weight/weight) ratio of polymer nanoparticles to metal oxide is for instance from about 0.1/1 to about 10.0/1 or from about 0.5/1 to about 10.0/1.

The continuous oil phase comprises for example an organic solvent, a silicone oil or a fluorinated oil. According to the invention “oil” means an organic phase immiscible with water. Organic solvents include hydrocarbons, for example, heptane, hexane, toluene, xylene, and the like, as well as alkanols such as methanol, ethanol, propanol, etc.

The emulsion droplets are collected, dried and the polymer is removed. Drying is performed for instance via microwave irradiation, in a thermal oven, under vacuum, in the presence of a desiccant, via spray-drying techniques or a combination thereof.

Polymer removal may be performed for example via calcination, pyrolysis or with a solvent (solvent removal). Calcination is performed in some embodiments at temperatures of at least about 200° C., at least about 500° C., at least about 1000° C., from about 200° C. to about 1200° C. or from about 200° C. to about 700° C. The calcining can be for a suitable period, e.g., from about 0.1 hour to about 12 hours or from about 1 hour to about 8.0 hours. In other embodiments, the calcining can be for at least about 0.1 hour, at least about 1 hour, at least about 5 hours or at least about 10 hours.

Alternatively, a liquid dispersion comprising polymer nanoparticles and a metal oxide is formed with an oil dispersed phase and a continuous water phase to form an oil-in-water emulsion. The oil droplets may be collected and dried as are aqueous droplets.

Alternatively, a liquid dispersion of polymer nanoparticles and a metal oxide is prepared and is spray-dried to form the polymer template microspheres without forming a liquid-in-liquid emulsion. In certain embodiments of spray-drying techniques, a liquid solution or dispersion is fed (e.g. pumped) to an atomizing nozzle associated with a compressed gas inlet. The feed is pumped through the atomizing nozzle to form liquid droplets. The droplets are surrounded by a pre-heated gas in an evaporation chamber, resulting in evaporation of solvent to produce solid particles. The dried particles are carried by the drying gas through a cyclone and deposited in a collection chamber. Gases include nitrogen and/or air. In an embodiment of a present spray-drying process, a liquid feed contains a water or oil phase, polymer particles and optionally metal oxide. Provided are polymer microspheres containing polymer nanospheres with optionally metal oxide in the interstitial spaces between the polymer nanospheres. The polymer nanospheres define the interstitial spaces. Spray-drying techniques include ink jet spray-drying methods and equipment.

In present spray-drying techniques, air may be considered a continuous phase with a dispersed liquid phase (a liquid-in-gas emulsion). In certain embodiments, spray-drying comprises an inlet temperature of from any of about 100° C., about 105° C., about 110° C., about 115° C., about 120° C., about 130° C., about 140° C., about 150° C., about 160° C. or about 170° C. to any of about 180° C., about 190° C., about 200° C., about 210° C., about 215° C. or about 220° C. In some embodiments a pump rate (feed flow rate) of from any of about 1 mL/min, about 2 mL/min, about 5 mL/min, about 6 mL/min, about 8 mL/min, about 10 mL/min, about 12 mL/min, about 14 mL/min or about 16 mL/min to any of about 18 mL/min, about 20 mL/min, about 22 mL/min, about 24 mL/min, about 26 mL/min, about 28 mL/min or about 30 mL/min is employed. Spray-drying techniques are disclosed for example in US2016/0170091.

FIG. 4 is a representation of a spray-drying process according to some embodiments of the invention.

In certain embodiments of spray-drying techniques, a feed solution or dispersion is fed to an atomizing nozzle associated with a compressed gas inlet. The feed is pumped through the atomizing nozzle to form liquid droplets. The droplets are surrounded by a pre-heated gas in an evaporation chamber, resulting in evaporation of solvent to produce solid particles. The dried particles are carried by the drying gas through a cyclone and deposited in a collection chamber. Gases include nitrogen and/or air. In a present spray-drying process, a liquid feed contains water, polymer nanoparticles and metal oxide.

The microspheres are spherical or spherical-like and are micron-scaled, for example having average diameters from about 0.5 microns (μm) to about 100 μm. The polymer nanoparticles employed as a template are also spherical and are nano-scaled, having average diameters for instance from about 50 nm to about 999 nm. The metal oxide employed may also be in particle form, which particles may be nano-scaled.

The metal oxide of the dispersion may be provided as metal oxide or may be provided from a metal oxide precursor, for instance via a sol-gel technique.

Drying of the polymer/metal oxide droplets and removal of the polymer provides microspheres having voids (pores). In general, in the present processes, each droplet provides a single microsphere. The pore diameters are dependent on the size of the polymer particles. Some “shrinkage” or compaction may occur upon removal of the polymer, providing pore sizes somewhat smaller than the original polymer particle size, for example from about 10% to about 40% smaller than the polymer particle size. The pore diameters vary as the polymer particle size varies (is polydisperse).

Pore diameters may range in some embodiments from about 50 nm to about 999 nm.

The average porosity of the present metal oxide microspheres may be relatively high, for example from about 0.10 or about 0.30 to about 0.80 or about 0.90. Average porosity of a microsphere means the total pore volume, as a fraction of the volume of the entire microsphere. Average porosity may be called “volume fraction.”

In some embodiments, a porous microsphere may have a solid core (center) where the porosity is in general towards the exterior surface of the microsphere. In other embodiments, a porous microsphere may have a hollow core where a major portion of the porosity is towards the interior of the microsphere. In other embodiments, the porosity may be distributed throughout the volume of the microsphere. In other embodiments, the porosity may exist as a gradient, with higher porosity towards the exterior surface of the microsphere and lower or no porosity (solid) towards the center; or with lower porosity towards the exterior surface and with higher or complete porosity (hollow) towards the center.

For any porous microsphere, the average microsphere diameter is larger than the average pore diameter, for example, the average microsphere diameter is at least about 25 times, at least about 30 times, at least about 35 times, or at least about 40 times larger than the average pore diameter.

In some embodiments, the ratio of average microsphere diameter to average pore diameter is for instance from any of about 40/1, about 50/1, about 60/1, about 70/1, about 80/1, about 90/1, about 100/1, about 110/1, about 120/1, about 130/1, about 140/1, about 150/1, about 160/1, about 170/1, about 180/1 or about 190/1 to any of about 200/1, about 210/1, about 220/1, about 230/1, about 240/1, about 250/1, about 260/1, about 270/1, about 280/1, about 290/1, about 300/1, about 310/1, about 320/1, about 330/1, about 340/1 or about 350/1.

Polymer template microspheres comprising polydisperse polymer nanospheres may provide, when the polymer is removed, metal oxide microspheres having pores that in general have varied pore diameters.

Without wishing to be bound by theory, it is believed that bulk samples of microspheres exhibit saturated color with reduced unwanted light scattering when porosity and/or microsphere diameter and/or pore diameter are within a certain range. Color properties of a bulk sample are important, as colorants are employed in bulk, for instance in a paint, an ink, a coating, a cosmetic or a material for a medical application or a security application. In some embodiments, white microspheres are desirable, for example for use as white colorants.

The porous microspheres comprise mainly metal oxide, that is, they may consist essentially of or consist of metal oxide. Advantageously, a bulk sample of the porous microspheres exhibits color observable by the human eye. A light absorber may also be present in the microspheres, which may provide a more saturated observable color. Absorbers include inorganic and organic pigments, for example a broadband absorber such as carbon black. Absorbers may for instance be added by physically mixing the microspheres and the absorbers together or by including the absorbers in the droplets to be dried. For carbon black, controlled calcination may be employed to produce carbon black in situ from polymer decomposition. A present microsphere may exhibit no observable color without added light absorber and exhibit observable color with added light absorber.

The porous microspheres may be employed as colorants for example for aqueous formulations, oil-based formulations, inks, coatings formulations, foods, plastics, cosmetics formulations or materials for medical applications or security applications. Coatings formulations include for instance automotive coatings, architectural coatings, varnishes, and the like.

The present porous metal oxide microspheres may exhibit angle-dependent color or angle-independent color. “Angle-dependent” color means that observed color has dependence on the angle of incident light on a sample or on the angle between the observer and the sample. “Angle-independent” color means that observed color has substantially no dependence on the angle of incident light on a sample or on the angle between the observer and the sample.

Angle-independent color may be achieved for example with the use of polydisperse polymer nanospheres. Angle-independent color may also be achieved when a step of drying the liquid droplets to provide polymer template microspheres is performed quickly, not allowing the polymer nanospheres to become ordered. Angle-dependent color may be achieved when a step of drying the liquid droplets is performed slowly.

For instance, the porous microspheres may comprise from about 60.0 wt % (weight percent) to about 99.9 wt % metal oxide and from about 0.1 wt % to about 40.0 wt % of one or more light absorbers, based on the total weight of the microspheres.

Also subject of the invention are polymer microspheres comprising polydisperse polymer nanospheres, methods for their preparation and compositions comprising them. The method comprises forming an aqueous dispersion of monodisperse polymer nanoparticles; forming at least one further aqueous dispersion of monodisperse polymer nanoparticles; providing a continuous oil phase; mixing the aqueous dispersions and the oil phase together to form a water-in-oil emulsion; forming emulsion droplets; and drying the emulsion droplets to provide polymer microspheres comprising polydisperse polymer nanospheres; wherein the average diameters of the monodisperse polymer nanoparticles of each of the dispersions are different.

Advantageously, the porous microspheres and polymer microspheres may be monodisperse.

According to the invention, particle size is synonymous with particle diameter and is determined for instance by scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Average particle size is synonymous with D50, meaning half of the population resides above this point, and half below. Particle size refers to primary particles. Particle size may be measured by laser light scattering techniques, with dispersions or dry powders.

Mercury porosimetry analysis was used to characterize the porosity of the microspheres. Mercury porosimetry applies controlled pressure to a sample immersed in mercury. External pressure is applied for the mercury to penetrate into the voids/pores of the material. The amount of pressure required to intrude into the voids/pores is inversely proportional to the size of the voids/pores. The mercury porosimeter generates volume and pore size distributions from the pressure versus intrusion data generated by the instrument using the Washburn equation. For example, porous silica microspheres containing voids/pores with an average size of 165 nm have an average porosity of 0.8.

The term “bulk sample” means a population of microspheres. For example, a bulk sample of microspheres is simply a bulk population of microspheres, for instance ≥0.5 mg, ≥0.7 mg, ≥1.0 mg, ≥2.5 mg, ≥5.0 mg, ≥10.0 mg or ≥25.0 mg. A bulk sample of microspheres may be substantially free of other components. The term “porous microspheres” may mean a bulk sample.

The phrase “exhibits color observable by the human eye” means color will be observed by an average person. This may be for any bulk sample distributed over any surface area, for instance a bulk sample distributed over a surface area of from any of about 1 cm², about 2 cm², about 3 cm², about 4 cm², about 5 cm² or about 6 cm² to any of about 7 cm², about 8 cm², about 9 cm², about 10 cm², about 11 cm², about 12 cm², about 13 cm², about 14 cm² or about 15 cm². It may also mean observable by a CIE 1931 2° standard observer and/or by a CIE 1964 10° standard observer. The background for color observation may be any background, for instance a white background, black background or a dark background anywhere between white and black.

The term “of” may mean “comprising”, for instance “a liquid dispersion of” may be interpreted as “a liquid dispersion comprising”.

The terms “microspheres”, “nanospheres”, “droplets”, etc., referred to herein may mean for example a plurality thereof, a collection thereof, a population thereof, a sample thereof or a bulk sample thereof.

The term “micro” or “micro-scaled” means from about 0.5 μm to about 999 μm. The term “nano” or “nano-scaled” means from about 1 nm to about 999 nm.

The terms “spheres” and “particles” may be interchangeable.

The term “monodisperse” in reference to a population of microspheres or nanospheres means particles having generally uniform shapes and generally uniform diameters. A present monodisperse population of microspheres or nanospheres for instance may have 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of the particles by number having diameters within ±7%, ±6%, ±5%, ±4%, ±3%, ±2% or ±1% of the average diameter of the population. The term “monodisperse polymer nanoparticles” refers to a population of monodisperse polymer nanoparticles.

The term “polydisperse” in reference to nanospheres means a sample comprising a first monodisperse population having a first average diameter and at least a second monodisperse population having a second average diameter, the first and second diameters being different. A polydisperse sample of microspheres contains at least two monodisperse populations and may contain 3, 4, 5, 6, etc. monodisperse populations, each having a different average particle size. A polydisperse sample having only first and second monodisperse polymer nanospheres is a “bimodal” sample, having a bimodal particle size distribution.

The term “substantially free of other components” means for example containing ≤5%, ≤4%, ≤3%, ≤2%, ≤1% or ≤0.5% by weight of other components. Likewise, the term “substantially no” means little or no.

A “substrate” may mean an aqueous-based or an oil-based substrate or “media”, which substrate may be a minor part or a major part of a final composition. A substrate may also mean a solid, a semi-solid, a gel, a liquid, a paste, a cream, etc.

Removal of a monodisperse population of polymer nanospheres provides porous metal oxide microspheres having a corresponding population of pores having an average pore diameter. Removal of more than one monodisperse population of polymer nanospheres (polydisperse polymer nanospheres) provides porous metal oxide microspheres having corresponding populations of pores having different average pore diameters. That is, porous metal oxide microspheres having more than one population of pores, each having an average pore diameter, wherein each population has a different average pore diameter and wherein the average pore diameters are from about 50 nm to about 999 nm.

The polymer nanosphere diameters of the polymer microspheres and the pore diameters of the porous microspheres may be for example bimodal, trimodal, quadrimodal, etc.

The articles “a” and “an” herein refer to one or to more than one (e.g. at least one) of the grammatical object. Any ranges cited herein are inclusive. The term “about” used throughout is used to describe and account for small fluctuations. For instance, “about” may mean the numeric value may be modified by ±5%, ±4%, ±3%, ±□2%, ±□1%, ±□0.5%, ±□0.4%, ±□0.3%, ±□0.2%, ±□0.1% or ±□0.05%. All numeric values are modified by the term “about” whether or not explicitly indicated. Numeric values modified by the term “about” include the specific identified value. For example “about 5.0” includes 5.0.

U.S. patents, U.S. patent applications and published U.S. patent applicants discussed herein are hereby incorporated by reference.

Unless otherwise indicated, all parts and percentages are by weight. Weight percent (wt %), if not otherwise indicated, is based on an entire composition free of any volatiles, that is, based on dry solids content.

A non-limiting first set of embodiments of the disclosure directed towards methods of preparation of polymer microspheres includes:

In a first embodiment, disclosed is method to prepare polymer microspheres comprising polydisperse polymer nanospheres, the method comprising forming a liquid solution or dispersion of monodisperse polymer nanoparticles; forming at least one further liquid solution or dispersion of monodisperse polymer nanoparticles; mixing each of the solutions or dispersions together; forming droplets of the mixture; and drying the droplets to provide polymer microspheres comprising polydisperse polymer nanospheres; wherein the average diameters of the monodisperse polymer nanoparticles of each of the solutions or dispersions are different.

Drying the droplets in some embodiments may comprise microwave irradiation, oven drying, drying under vacuum, drying in the presence of a desiccant or a combination thereof.

In a second embodiment, a method according to embodiment 1, comprising mixing the solutions or dispersions together and spray-drying the mixture to provide the polymer microspheres. In a third embodiment, a method according to embodiment 1, comprising forming the liquid droplets with a vibrating nozzle. In a fourth embodiment, a method according to embodiments 1 to 3, wherein the liquid droplets are aqueous droplets. In a fifth embodiment, a method according to embodiments 1 to 3, wherein the liquid droplets are oil droplets.

In a sixth embodiment, a method according to embodiment 1, comprising providing a continuous phase and mixing the solutions or dispersions with the continuous phase to form an emulsion containing dispersed liquid solution or dispersion droplets.

In a seventh embodiment, a method according to embodiment 6, comprising providing a continuous oil phase and mixing aqueous solutions or dispersions with the continuous oil phase to form a water-in-oil emulsion containing aqueous droplets. In an eighth embodiment, a method according to embodiment 6, comprising providing a continuous aqueous phase and mixing oil solutions or dispersions with the continuous phase to form an oil-in-water emulsion containing oil droplets. In a ninth embodiment, a method according to embodiments 6 to 8, comprising collecting the droplets.

In a tenth embodiment, a method according to embodiment 9, comprising drying the droplets to provide polymer microspheres comprising polydisperse polymer nanospheres. In an eleventh embodiment, a method according to embodiments 6 to 10, wherein drying the droplets comprises microwave irradiation, oven drying, drying under vacuum, drying in the presence of a desiccant or a combination thereof. In a twelfth embodiment, a method according to embodiments 7 to 11, wherein the oil phase or solution or dispersion comprises a hydrocarbon, a silicone oil or a fluorinated oil. In a thirteenth embodiment, a method according to embodiments 6 to 12, wherein forming the droplets occurs in a microfluidic device.

In a fourteenth embodiment, a method according to embodiments 6 to 13, wherein forming the droplets occurs in a microfluidic device which contains a droplet junction having a channel width of from any of about 10 μm, about 15 μm, about 20 μm, about 25 μm, about 30 μm, about 35 μm, about 40 μm or about 45 μm to any of about 50 μm, about 55 μm, about 60 μm, about 65 μm, about 70 μm, about 75 μm, about 80 μm, about 85 μm, about 90 μm, about 95 μm or about 100 μm. In a fifteenth embodiment, a method according to embodiments 13 or 14, comprising collecting the emulsion droplets from the microfluidic device.

In a sixteenth embodiment, a method according to any of the preceding embodiments, comprising forming a first liquid solution or dispersion of first monodisperse polymer nanoparticles and forming a second liquid solution or dispersion of second monodisperse polymer nanoparticles; for example wherein the wt/wt ratio of the first monodisperse polymer nanoparticles to the second monodisperse polymer nanoparticles is from any of about 1/20, about 1/19, about 1/18, about 1/17, about 1/16, about 1/15, about 1/14, about 1/13, about 1/12, about 1/11, about 1/10, about 1/9, about 1/8, about 1/7, about 1/6, about 1/5, about 1/4, about 1/3, about 1/2, or about 1/1 to any of about 2/1, about 3/1, about 4/1, about 5/1, about 6/1, about 7/1, about 8/1, about 9/1, about 10/1, about 11/1, about 12/1, about 13/1, about 14/1, about 15/1, about 16/1, about 17/1, about 18/1, about 19/1 or about 20/1.

In a seventeenth embodiment, a method according to any of the preceding embodiments, wherein the polymer nanoparticles have an average diameter of from any of about 50 nm, about 75 nm, about 100 nm, about 130 nm, about 160 nm, about 190 nm, about 210 nm, about 240 nm, about 270 nm, about 300 nm, about 330 nm, about 360 nm, about 390 nm, about 410 nm, about 440 nm, about 470 nm, about 500 nm, about 530 nm, about 560 nm, about 590 nm or about 620 nm to any of about 650 nm, about 680 nm, about 710 nm, about 740 nm, about 770 nm, about 800 nm, about 830 nm, about 860 nm, about 890 nm, about 910 nm, about 940 nm, about 970 nm or about 990 nm.

In an eighteenth embodiment, a method according to any of the preceding embodiments, wherein the polymers are selected from the group consisting of poly(meth)acrylic acid, poly(meth)acrylates, polystyrenes, polyacrylamides, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, derivatives thereof, salts thereof, copolymers thereof and combinations thereof. In a nineteenth embodiment, a method according to any of the preceding embodiments, wherein the polymers are selected from the group consisting of polystyrenes, for example polystyrene copolymers such as polystyrene/acrylic acid, polystyrene/poly(ethylene glycol) methacrylate or polystyrene/styrene sulfonate.

In a twentieth embodiment, a method according to any of the preceding embodiments, wherein the microspheres have an average diameter of from about 0.5 μm to about 100 μm. In a twenty-first embodiment, a method according to any of the preceding embodiments, wherein the microspheres have an average diameter of from about 1 μm to about 75 μm, about 2 μm to about 70 μm, from about 3 μm to about 65 μm, from about 4 μm to about 60 μm, from about 5 μm to about 55 μm or from about 5 μm to about 50 μm; for example from any of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm or about 15 μm to any of about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, about 21 μm, about 22 μm, about 23 μm, about 24 μm or about 25 μm. In a twenty-second embodiment, a method according to any of the preceding embodiments, wherein the microspheres have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm.

In a twenty-third embodiment, a method according to any of the preceding embodiments, wherein a bulk sample of the polymer microspheres exhibits color observable by the human eye. In a twenty-fourth embodiment, a method according to any of the preceding embodiments, wherein a bulk sample of the polymer microspheres exhibits angle-independent color observable by the human eye. In a twenty-fifth embodiment, a method according to any of embodiments 1-23, wherein a bulk sample of the polymer microspheres exhibits angle-dependent color observable by the human eye.

In a twenty-sixth embodiment, a method according to any of the preceding embodiments, wherein the microspheres are monodisperse. In a twenty-seventh embodiment, a method according to any of the preceding embodiments, wherein the polymer microspheres are a bulk sample of microspheres.

In a twenty-eighth embodiment, a method according to any of the preceding embodiments, comprising adding a metal oxide to one or more of the liquid solutions or dispersions; or adding a metal oxide to the mixture.

In a twenty-ninth embodiment, polymer microspheres prepared according to any of the preceding methods. In a thirtieth embodiment, a bulk sample of polymer microspheres prepared according to any of the preceding methods.

A non-limiting second set of embodiments of the invention directed towards polymer microspheres includes:

Polymer microspheres comprising more than one population of monodisperse polymer nanospheres, wherein each population of monodisperse polymer nanospheres has different average diameters.

In a second embodiment, polymer microspheres according to embodiment 1, comprising a first population of monodisperse polymer nanoparticles and a second population of monodisperse polymer nanoparticles; for example wherein the wt/wt ratio of the first population of polymer nanospheres to the second population of polymer nanospheres is from any of about 1/20, about 1/19, about 1/18, about 1/17, about 1/16, about 1/15, about 1/14, about 1/13, about 1/12, about 1/11, about 1/10, about 1/9, about 1/8, about 1/7, about 1/6, about 1/5, about 1/4, about 1/3, about 1/2, or about 1/1 to any of about 2/1, about 3/1, about 4/1, about 5/1, about 6/1, about 7/1, about 8/1, about 9/1, about 10/1, about 11/1, about 12/1, about 13/1, about 14/1, about 15/1, about 16/1, about 17/1, about 18/1, about 19/1 or about 20/1.

In a third embodiment, polymer microspheres according to embodiments 1 or 2, wherein the polymer nanospheres have an average diameter of from any of about 100 nm, about 130 nm, about 160 nm, about 190 nm, about 210 nm, about 240 nm, about 270 nm, about 300 nm, about 330 nm, about 360 nm, about 390 nm, about 410 nm, about 440 nm, about 470 nm, about 500 nm, about 530 nm, about 560 nm, about 590 nm or about 620 nm to any of about 650 nm, about 680 nm, about 710 nm, about 740 nm, about 770 nm, about 800 nm, about 830 nm, about 860 nm, about 890 nm, about 910 nm, about 940 nm or about 970 nm.

In a fourth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the polymers are selected from the group consisting of poly(meth)acrylic acid, poly(meth)acrylates, polystyrenes, polyacrylamides, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, derivatives thereof, salts thereof, copolymers thereof and combinations thereof.

In a fifth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the polymers are selected from the group consisting of polystyrenes, for example polystyrene copolymers such as polystyrene/acrylic acid, polystyrene/poly(ethylene glycol) methacrylate or polystyrene/styrene sulfonate.

In a sixth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the polymers of each population of polymer nanospheres are identical. In a seventh embodiment, polymer microspheres according to any of embodiments 1 to 5, wherein the polymers of each population of polymer nanospheres are different.

In an eighth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the microspheres have an average diameter of from about 0.5 μm to about 100 μm. In a ninth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the microspheres have an average diameter of from about 1 μm to about 75 μm, about 2 μm to about 70 μm, from about 3 μm to about 65 μm, from about 4 μm to about 60 μm, from about 5 μm to about 55 μm or from about 5 μm to about 50 μm; for example from any of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm or about 15 μm to any of about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, about 21 μm, about 22 μm, about 23 μm, about 24 μm or about 25 μm. In a tenth embodiment, polymer microspheres according to any of the preceding embodiments, wherein the microspheres have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm.

In a twelfth embodiment, polymer microspheres according to any of the preceding embodiments, wherein a bulk sample of the polymer microspheres exhibits color observable by the human eye. In a thirteenth embodiment, polymer microspheres according to any of the preceding embodiments, wherein a bulk sample of the polymer microspheres exhibits angle-independent color observable by the human eye. In a fourteenth embodiment, polymer microspheres according to any of embodiments 1-12, wherein a bulk sample of the polymer microspheres exhibits angle-dependent color observable by the human eye.

In a fifteenth embodiment, polymer microspheres according to any of the preceding embodiments, which are monodisperse.

In a sixteenth embodiment, polymer microspheres according to any of the preceding embodiments, further comprising a metal oxide.

A non-limiting third set of embodiments of the invention directed towards methods of preparation of porous metal oxide microspheres includes:

In a first embodiment, disclosed is a method to prepare porous metal oxide microspheres, the method comprising forming a liquid solution or dispersion of monodisperse polymer nanoparticles; forming at least one further liquid solution or dispersion of monodisperse polymer nanoparticles; mixing each of the solutions or dispersions together; wherein a metal oxide is added to one or more of the solutions or suspensions and/or wherein a metal oxide is added to the mixture to form a liquid dispersion comprising polymer nanoparticles and metal oxide; forming liquid droplets of the liquid dispersion; drying the droplets to provide polymer template microspheres comprising polydisperse polymer nanospheres and metal oxide; and removing the polymer nanospheres from the template microspheres to provide the porous metal oxide microspheres; wherein the average diameters of the monodisperse polymer nanoparticles of each of the solutions or dispersions are different.

In a second embodiment, a method according to embodiment 1, comprising mixing the solutions or dispersions together and spray-drying the mixture to provide the polymer template microspheres and removing the polymer nanospheres from the template microspheres.

In a third embodiment, a method according to embodiment 1, comprising forming the liquid droplets with a vibrating nozzle. In a fourth embodiment, a method according to embodiments 1 to 3, wherein the liquid droplets are aqueous droplets. In a fifth embodiment, a method according to embodiments 1 to 3, wherein the liquid droplets are oil droplets.

In a sixth embodiment, a method according to embodiment 1, comprising providing a continuous phase and mixing the liquid dispersion with the continuous phase to form an emulsion containing dispersed liquid dispersion droplets. In a seventh embodiment, a method according to embodiment 6, comprising providing a continuous oil phase and mixing an aqueous dispersion with the continuous oil phase to form a water-in-oil emulsion containing aqueous droplets.

In an eighth embodiment, a method according to embodiment 6, comprising providing a continuous aqueous phase and mixing an oil dispersion with the continuous phase to form an oil-in-water emulsion containing oil droplets. In a ninth embodiment, a method according to embodiments 6 to 8, comprising collecting the droplets. In a tenth embodiment, a method according to embodiment 9, comprising drying the droplets to provide polymer template microspheres comprising polydisperse polymer nanospheres.

In an eleventh embodiment, a method according to embodiments 6 to 10, wherein drying the droplets comprises microwave irradiation, oven drying, drying under vacuum, drying in the presence of a desiccant or a combination thereof.

In a twelfth embodiment, a method according to embodiments 7 to 11, wherein the oil phase or solution or dispersion comprises a hydrocarbon, a silicone oil or a fluorinated oil.

In a thirteenth embodiment, a method according to embodiments 6 to 12, wherein forming the droplets occurs in a microfluidic device. In a fourteenth embodiment, a method according to embodiments 6 to 13, wherein forming the droplets occurs in a microfluidic device which contains a droplet junction having a channel width of from any of about 10 μm, about 15 μm, about 20 μm, about 25 μm, about 30 μm, about 35 μm, about 40 μm or about 45 μm to any of about 50 μm, about 55 μm, about 60 μm, about 65 μm, about 70 μm, about 75 μm, about 80 μm, about 85 μm, about 90 μm, about 95 μm or about 100 μm. In a fifteenth embodiment, a method according to embodiments 13 or 14, comprising collecting the emulsion droplets from the microfluidic device.

In a sixteenth embodiment, a method according to any of the preceding embodiments, comprising forming a first liquid solution or dispersion of first monodisperse polymer nanoparticles and forming a second liquid solution or dispersion of second monodisperse polymer nanoparticles; for example wherein the wt/wt ratio of the first monodisperse polymer nanoparticles to the second monodisperse polymer nanoparticles is from any of about 1/20, about 1/19, about 1/18, about 1/17, about 1/16, about 1/15, about 1/14, about 1/13, about 1/12, about 1/11, about 1/10, about 1/9, about 1/8, about 1/7, about 1/6, about 1/5, about 1/4, about 1/3, about 1/2, or about 1/1 to any of about 2/1, about 3/1, about 4/1, about 5/1, about 6/1, about 7/1, about 8/1, about 9/1, about 10/1, about 11/1, about 12/1, about 13/1, about 14/1, about 15/1, about 16/1, about 17/1, about 18/1, about 19/1 or about 20/1.

In a seventeenth embodiment, a method according to any of the preceding embodiments, wherein the polymer nanoparticles have an average diameter of from any of about 50 nm, about 75 nm, about 100 nm, about 130 nm, about 160 nm, about 190 nm, about 210 nm, about 240 nm, about 270 nm, about 300 nm, about 330 nm, about 360 nm, about 390 nm, about 410 nm, about 440 nm, about 470 nm, about 500 nm, about 530 nm, about 560 nm, about 590 nm or about 620 nm to any of about 650 nm, about 680 nm, about 710 nm, about 740 nm, about 770 nm, about 800 nm, about 830 nm, about 860 nm, about 890 nm, about 910 nm, about 940 nm, about 970 nm or about 990 nm.

In an eighteenth embodiment, a method according to any of the preceding embodiments, wherein the polymer is selected from the group consisting of poly(meth)acrylic acid, poly(meth)acrylates, polystyrenes, polyacrylamides, polyethylene, polypropylene, polylactic acid, polyacrylonitrile, derivatives thereof, salts thereof, copolymers thereof and combinations thereof. In a nineteenth embodiment, a method according to any of the preceding embodiments, wherein the polymers are selected from the group consisting of polystyrenes, for example polystyrene copolymers such as polystyrene/acrylic acid, polystyrene/poly(ethylene glycol) methacrylate or polystyrene/styrene sulfonate.

In a twentieth embodiment, a method according to any of the preceding embodiments, wherein the metal oxide is one or more of silica, titania, alumina, zirconia, ceria, iron oxides, zinc oxide, indium oxide, tin oxide or chromium oxide.

In a twenty-first embodiment, a method according to any of the preceding embodiments, wherein the wt/wt ratio of polymer nanoparticles in total to the metal oxide is from any of about 0.1/1, 0.5/1, about 1.0/1, about 1.5/1, about 2.0/1, about 2.5/1 or about 3.0/1 to any of about 3.5/1, about 4.0/1, about 5.0/1, about 5.5/1, about 6.0/1, about 6.5/1, about 7.0/1, about 8.0/1, about 9.0/1 or about 10.0/1.

In a twenty-second embodiment, a method according to any of the preceding embodiments wherein drying the droplets comprises microwave irradiation, oven drying, drying under vacuum, drying in the presence of a desiccant, or a combination thereof.

In a twenty-third embodiment, a method according to any of the preceding embodiments, wherein removing the polymer nanospheres comprises calcining the template microspheres at temperatures of from any of about 200° C., about 350° C., from about 400° C., from about 450° C., about 500° C. or about 550° C. to any of about 600° C., about 650° C., about 700° C. or about 1200° C. for a period of from any of about 0.1 h (hour), 1 h, about 1.5 h, about 2.0 h, about 2.5 h, about 3.0 h, about 3.5 h or about 4.0 h to any of about 4.5 h, about 5.0 h, about 5.5 h, about 6.0 h, about 6.5 h, about 7.0 h, about 7.5 h, about 8.0 h or about 12 h. Alternatively, the calcining can be at temperatures of at least about 200° C., at least about 500° C., or at least about 1000° C., for a suitable period, e.g., for at least about 0.1 hour, at least about 1 hour, at least about 5 hours or at least about 10 hours.

In a twenty-fourth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average diameter of from about 0.5 μm to about 100 μm and an average porosity of from about 0.10 to about 0.90 or from about 0.10 to about 0.80; wherein the porous microspheres have more than one population of pores each having an average pore diameter, wherein each population has a different average pore diameter; and wherein the average pore diameters are from about 50 nm to about 999 nm; for example wherein the porous microspheres have a first population of pores having an average pore diameter of from about 50 nm to about 999 nm and a second population of pores having average pore diameter of from about 50 nm to about 999 nm, wherein the first and second average pore diameters are different.

In a twenty-fifth embodiment, a method according to any of the preceding claims, wherein the porous microspheres have an average diameter of from about 1 μm to about 75 μm, about 2 μm to about 70 μm, from about 3 μm to about 65 μm, from about 4 μm to about 60 μm, from about 5 μm to about 55 μm or from about 5 μm to about 50 μm; for example from any of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm or about 15 μm to any of about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, about 21 μm, about 22 μm, about 23 μm, about 24 μm or about 25 μm.

In a twenty-sixth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average porosity of from any of about 0.10, about 0.12, about 0.14, about 0.16, about 0.18, about 0.20, about 0.22, about 0.24, about 0.26, about 0.28, about 0.30, about 0.32, about 0.34, about 0.36, about 0.38, about 0.40, about 0.42, about 0.44, about 0.46, about 0.48 about 0.50, about 0.52, about 0.54, about 0.56, about 0.58 or about 0.60 to any of about 0.62, about 0.64, about 0.66, about 0.68, about 0.70, about 0.72, about 0.74, about 0.76, about 0.78, about 0.80 or about 0.90.

In a twenty-seventh embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average pore diameter of from any of about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 100 nm, about 120 nm, about 140 nm, about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm, about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm, about 360 nm, about 380 nm, about 400 nm, about 420 nm or about 440 nm to any of about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm, about 560 nm, about 580 nm, about 600 nm, about 620 nm, about 640 nm, about 660 nm, about 680 nm, about 700 nm, about 720 nm, about 740 nm, about 760 nm, about 780 nm or about 800 nm.

In a twenty-eighth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm.

In a twenty-ninth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65.

In a thirtieth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average pore diameter of from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In a thirty-first embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm; an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65; and an average pore diameter of from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In a thirty-second embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres comprise from about 60.0 wt % to about 99.9 wt % metal oxide, for example comprising from any of about 60.0 wt %, about 64.0 wt %, about 67.0 wt %, about 70.0 wt %, about 73.0 wt %, about 76.0 wt %, about 79.0 wt %, about 82.0 wt % or about 85.0 wt % to any of about 88.0 wt %, about 91.0 wt %, about 94.0 wt %, about 97.0 wt %, about 98.0 wt %, about 99.0 wt % or about 99.9 wt % metal oxide, based on the total weight of the microspheres.

In a thirty-third embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres comprise from about 0.1 wt % to about 40.0 wt % of one or more light absorbers, for example comprising from any of about 0.1 wt %, about 0.3 wt %, about 0.5 wt %, about 0.7 wt %, about 0.9 wt %, about 1.0 wt %, about 1.5 wt %, about 2.0 wt %, about 2.5 wt %, about 5.0 wt %, about 7.5 wt %, about 10.0 wt %, about 13.0 wt %, about 17.0 wt %, about 20.0 wt % or about 22.0 wt % to any of about 24.0 wt %, about 27.0 wt %, about 29.0 wt %, about 31.0 wt %, about 33.0 wt %, about 35.0 wt %, about 37.0 wt %, about 39.0 wt % or about 40.0 wt % of one or more light absorbers, based on the total weight of the microspheres. In a thirty-fourth embodiment, a method according to any of the preceding embodiments, wherein the porous microspheres comprise one or more light absorbers selected from the group consisting of inorganic and organic pigments, for example carbon black.

In a thirty-fifth embodiment, a method according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits color observable by the human eye. In a thirty-sixth embodiment, a method according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits angle-independent color observable by the human eye. In a thirty-seventh embodiment, a method according to any of embodiments 1-35, wherein a bulk sample of the porous microspheres exhibits angle-dependent color observable by the human eye.

In a thirty-eighth embodiment, a method according to any of the preceding embodiments, wherein the microspheres are monodisperse. In a thirty-ninth embodiment, a method according to any of the preceding embodiments, wherein the porous metal oxide microspheres are a bulk sample of microspheres.

In a fortieth embodiment, porous microspheres prepared according to any of the preceding methods. In a forty-first embodiment, a bulk sample of microspheres prepared according to any of the preceding methods.

A non-limiting fourth set of embodiments of the disclosure directed towards porous metal oxide microspheres includes:

In a first embodiment, disclosed are porous microspheres comprising a metal oxide, wherein the microspheres have an average diameter of from about 0.5 μm to about 100 μm and an average porosity of from about 0.10 to about 0.80; wherein the porous microspheres have more than one population of pores each having an average pore diameter, wherein each population has a different average pore diameter; and wherein the average pore diameters are from about 50 nm to about 999 nm; for example, wherein the microspheres have a first population of pores having an average pore diameter of from about 50 nm to about 999 nm and a second population of pores having an average pore diameter of from about 50 nm to about 999 nm, wherein the first and second average pore diameters are different.

In a second embodiment, porous microspheres according to embodiment 1, which have an average diameter of from about 1 μm to about 75 μm, about 2 μm to about 70 μm, from about 3 μm to about 65 μm, from about 4 μm to about 60 μm, from about 5 μm to about 55 μm or from about 5 μm to about 50 μm; for example from any of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm or about 15 μm to any of about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, about 21 μm, about 22 μm, about 23 μm, about 24 μm or about 25 μm.

In a third embodiment, porous microspheres according to embodiments 1 or 2, which have an average porosity of from any of about 0.10, about 0.12, about 0.14, about 0.16, about 0.18, about 0.20, about 0.22, about 0.24, about 0.26, about 0.28, about 0.30, about 0.32, about 0.34, about 0.36, about 0.38, about 0.40, about 0.42, about 0.44, about 0.46, about 0.48 about 0.50, about 0.52, about 0.54, about 0.56, about 0.58 or about 0.60 to any of about 0.62, about 0.64, about 0.66, about 0.68, about 0.70, about 0.72, about 0.74, about 0.76, about 0.78, about 0.80 or about 0.90.

In a fourth embodiment, porous microspheres according to any of the preceding embodiments, wherein the average pore diameters are from any of about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 100 nm, about 120 nm, about 140 nm, about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm, about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm, about 360 nm, about 380 nm, about 400 nm, about 420 nm or about 440 nm to any of about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm, about 560 nm, about 580 nm, about 600 nm, about 620 nm, about 640 nm, about 660 nm, about 680 nm, about 700 nm, about 720 nm, about 740 nm, about 760 nm, about 780 nm or about 800 nm.

In a fifth embodiment, porous microspheres according to any of the preceding embodiments, which have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm.

In a sixth embodiment, porous microspheres according to any of the preceding embodiments, which have an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65.

In a seventh embodiment, porous microspheres according to any of the preceding embodiments, wherein the average pore diameters are from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In an eighth embodiment, porous microspheres according to any of the preceding embodiments, which have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm; and which have an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65; and which have average pore diameters of from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In a ninth embodiment, porous microspheres according to any of the preceding embodiments, comprising from about 60.0 wt % to about 99.9 wt % metal oxide, for example comprising from any of about 60.0 wt %, about 64.0 wt %, about 67.0 wt %, about 70.0 wt %, about 73.0 wt %, about 76.0 wt %, about 79.0 wt %, about 82.0 wt % or about 85.0 wt % to any of about 88.0 wt %, about 91.0 wt %, about 94.0 wt %, about 97.0 wt %, about 98.0 wt %, about 99.0 wt % or about 99.9 wt % metal oxide, based on the total weight of the microspheres.

In a tenth embodiment, porous microspheres according to any of the preceding embodiments, wherein the metal oxide is selected from the group consisting of silica, titania, alumina, zirconia, ceria, iron oxides, zinc oxide, indium oxide, tin oxide, chromium oxide and combinations thereof. In an eleventh embodiment, porous microspheres according to any of the preceding embodiments, wherein the metal oxide is selected from the group consisting of silica, titania, alumina and combinations thereof.

In a twelfth embodiment, porous microspheres according to any of the preceding embodiments, comprising from about 0.1 wt % to about 40.0 wt % of one or more light absorbers, for example comprising from any of about 0.1 wt %, about 0.3 wt %, about 0.5 wt %, about 0.7 wt %, about 0.9 wt %, about 1.0 wt %, about 1.5 wt %, about 2.0 wt %, about 2.5 wt %, about 5.0 wt %, about 7.5 wt %, about 10.0 wt %, about 13.0 wt %, about 17.0 wt %, about 20.0 wt % or about 22.0 wt % to any of about 24.0 wt %, about 27.0 wt %, about 29.0 wt %, about 31.0 wt %, about 33.0 wt %, about 35.0 wt %, about 37.0 wt %, about 39.0 wt % or about 40.0 wt % of one or more light absorbers, based on the total weight of the microspheres.

In a thirteenth embodiment, porous microspheres according to any of the preceding embodiments, comprising one or more light absorbers selected from the group consisting of inorganic and organic pigments, for example carbon black.

In a fourteenth embodiment, porous microspheres according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits color observable by the human eye. In a fifteenth embodiment, porous microspheres according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits angle-independent color observable by the human eye. In a sixteenth embodiment, porous microspheres according to any of embodiments 1-14, wherein a bulk sample of the porous microspheres exhibits angle-dependent color observable by the human eye.

In a seventeenth embodiment, porous microspheres according to any of the preceding embodiments, wherein the microspheres are monodisperse.

In an eighteenth embodiment, a composition comprising a substrate and the porous microspheres according to any of the preceding embodiments. In a nineteenth embodiment, a composition according to embodiment 18, which is an aqueous formulation, an oil-based formulation, a coating formulation, a food, an ink, a plastic, a cosmetic formulation or a material for a medical application or a security application.

A non-limiting fifth set of embodiments of the disclosure directed towards metal oxide microspheres includes:

In a first embodiment, porous microspheres comprising a metal oxide, wherein a bulk sample of the porous microspheres exhibits color observable by the human eye; wherein the porous microspheres have more than one population of pores each having an average pore diameter, wherein each population has a different average pore diameter.

In a second embodiment, porous microspheres according to embodiment 1, wherein the microspheres have an average diameter of from about 0.5 μm to about 100 μm and an average porosity of from about 0.10 to about 0.90 or from about 0.10 to about 0.80; wherein the porous microspheres have more than one population of pores each population having an average pore diameter, wherein each population has a different average pore diameter; and wherein the average pore diameters are from about 50 nm to about 999 nm; for example, wherein the microspheres have a first population of pores having an average pore diameter of from about 50 nm to about 999 nm and a second population of pores having an average pore diameter of from about 50 nm to about 999 nm, wherein the first and second average pore diameters are different.

In a third embodiment, porous microspheres according to embodiments 1 or 2, which have an average diameter of from about 1 μm to about 75 μm, about 2 μm to about 70 μm, from about 3 μm to about 65 μm, from about 4 μm to about 60 μm, from about 5 μm to about 55 μm or from about 5 μm to about 50 μm; for example from any of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm or about 15 μm to any of about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, about 21 μm, about 22 μm, about 23 μm, about 24 μm or about 25 μm.

In a fourth embodiment, porous microspheres according to any of the preceding embodiments, which have an average porosity of from any of about 0.10, about 0.12, about 0.14, about 0.16, about 0.18, about 0.20, about 0.22, about 0.24, about 0.26, about 0.28, about 0.30, about 0.32, about 0.34, about 0.36, about 0.38, about 0.40, about 0.42, about 0.44, about 0.46, about 0.48 about 0.50, about 0.52, about 0.54, about 0.56, about 0.58 or about 0.60 to any of about 0.62, about 0.64, about 0.66, about 0.68, about 0.70, about 0.72, about 0.74, about 0.76, about 0.78, about 0.80 or about 0.90.

In a fifth embodiment, porous microspheres according to any of the preceding embodiments, which have average pore diameters of from any of about 50 nm, about 60 nm, about 70 nm, about 80 nm, about 100 nm, about 120 nm, about 140 nm, about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm, about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm, about 360 nm, about 380 nm, about 400 nm, about 420 nm or about 440 nm to any of about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm, about 560 nm, about 580 nm, about 600 nm, about 620 nm, about 640 nm, about 660 nm, about 680 nm, about 700 nm, about 720 nm, about 740 nm, about 760 nm, about 780 nm or about 800 nm.

In a sixth embodiment, porous microspheres according to any of the preceding embodiments, which have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm.

In a seventh embodiment, porous microspheres according to any of the preceding embodiments, which have an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65.

In an eighth embodiment, porous microspheres according to any of the preceding embodiments, which have average pore diameters of from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In a ninth embodiment, porous microspheres according to any of the preceding embodiments, which have an average diameter of from any of about 4.5 μm, about 4.8 μm, about 5.1 μm, about 5.4 μm, about 5.7 μm, about 6.0 μm, about 6.3 μm, about 6.6 μm, about 6.9 μm, about 7.2 μm or about 7.5 μm to any of about 7.8 μm about 8.1 μm, about 8.4 μm, about 8.7 μm, about 9.0 μm, about 9.3 μm, about 9.6 μm or about 9.9 μm; and which have an average porosity of from any of about 0.45, about 0.47, about 0.49, about 0.51, about 0.53, about 0.55 or about 0.57 to any of about 0.59, about 0.61, about 0.63 or about 0.65; and which have average pore diameters of from any of about 220 nm, about 225 nm, about 230 nm, about 235 nm, about 240 nm, about 245 nm or about 250 nm to any of about 255 nm, about 260 nm, about 265 nm, about 270 nm, about 275 nm, about 280 nm, about 285 nm, about 290 nm, about 295 nm or about 300 nm.

In a tenth embodiment, porous microspheres according to any of the preceding embodiments, comprising from about 60.0 wt % to about 99.9 wt % metal oxide, for example comprising from any of about 60.0 wt %, about 64.0 wt %, about 67.0 wt %, about 70.0 wt %, about 73.0 wt %, about 76.0 wt %, about 79.0 wt %, about 82.0 wt % or about 85.0 wt % to any of about 88.0 wt %, about 91.0 wt %, about 94.0 wt %, about 97.0 wt %, about 98.0 wt %, about 99.0 wt % or about 99.9 wt % metal oxide, based on the total weight of the microspheres.

In an eleventh embodiment, porous microspheres according to any of the preceding embodiments, wherein the metal oxide is selected from the group consisting of silica, titania, alumina, zirconia, ceria, iron oxides, zinc oxide, indium oxide, tin oxide, chromium oxide and combinations thereof. In a twelfth embodiment, porous microspheres according to any of the preceding embodiments, wherein the metal oxide is selected from the group consisting of silica, titania, alumina and combinations thereof.

In a thirteenth embodiment, porous microspheres according to any of the preceding embodiments, comprising from about 0.1 wt % to about 40.0 wt % of one or more light absorbers, for example comprising from any of about 0.1 wt %, about 0.3 wt %, about 0.5 wt %, about 0.7 wt %, about 0.9 wt %, about 1.0 wt %, about 1.5 wt %, about 2.0 wt %, about 2.5 wt %, about 5.0 wt %, about 7.5 wt %, about 10.0 wt %, about 13.0 wt %, about 17.0 wt %, about 20.0 wt % or about 22.0 wt % to any of about 24.0 wt %, about 27.0 wt %, about 29.0 wt %, about 31.0 wt %, about 33.0 wt %, about 35.0 wt %, about 37.0 wt %, about 39.0 wt % or about 40.0 wt % of one or more light absorbers, based on the total weight of the microspheres. In a fourteenth embodiment, porous microspheres according to any of the preceding embodiments, comprising one or more light absorbers selected from the group consisting of inorganic and organic pigments, for example carbon black.

In a fifteenth embodiment, porous microspheres according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits color observable by the human eye.

In a sixteenth embodiment, porous microspheres according to any of the preceding embodiments, wherein the microspheres are monodisperse.

In an seventeenth embodiment, porous microspheres according to any of the preceding embodiments, wherein a bulk sample of the porous microspheres exhibits angle-independent color observable by the human eye. In an eighteenth embodiment, porous microspheres according to any of embodiments 1-16, wherein a bulk sample of the porous microspheres exhibits angle-dependent color observable by the human eye.

In a nineteenth embodiment, a composition comprising a substrate and the porous microspheres according to any of the preceding embodiments. In a twentieth embodiment, a composition according to embodiment 19, which is an aqueous formulation, an oil-based formulation, a coating formulation, a food, an ink, a plastic, a cosmetic formulation or a material for a medical application or a security application.

EXAMPLES Example 1 Polymer Microspheres

A styrene/acrylic acid copolymer is prepared as follows: 230 mL deionized (DI) water is added to a 3-neck reaction flask equipped with a thermometer, condenser, magnetic stirring and nitrogen atmosphere. The water is heated to 80° C. and 10 g of styrene are added with stirring, followed by 100 mg acrylic acid dissolved in 10 mL DI water via syringe. 100 mg of ammonium persulfate is dissolved in 10 mL DI water and added to the stirred mixture via syringe. The reaction mixture is stirred for 24 hours at 80° C. The polymer colloid dispersion is allowed to cool to room temperature and is purified via centrifugation, producing polystyrene nanospheres having an average particle size of 250 nm.

Similarly, a styrene/acrylic acid copolymer is prepared to produce polystyrene nanospheres having an average particle size of 350 nm.

The first aqueous polystyrene colloid dispersion (250 nm) is mixed with the second aqueous polystyrene colloid dispersion (350 nm) in a wt/wt ratio of 7/3 and the mixture is diluted to 1 wt % with deionized water and is sonicated to prevent particle agglomeration. A continuous oil phase contains 0.1 wt % polyethylene glycol/perfluoropolyether surfactant in a fluorinated oil. The aqueous colloid dispersion mixture and oil are each injected into a microfluidic device having a 50 μm droplet junction via syringes associated with pumps. The system is allowed to equilibrate until monodisperse droplets are produced. The monodisperse droplets are collected in a reservoir.

Collected droplets are dried in an oven at 45° C. for 4 hours to provide monodisperse polymer microspheres. The monodisperse polystyrene microspheres comprise polystyrene nanospheres having a bimodal particle size distribution.

Example 2 Porous Metal Oxide Microspheres

Example 1 is repeated, adding 1 wt % silica nanoparticles to the aqueous mixture of first and second colloid dispersions prior to mixing with the oil phase to form a water-in-oil emulsion. Collected droplets from the microfluidic device are dried as in Example 1 to form polymer template microspheres. The polymer template microspheres are calcined by placing on a silicon wafer, heating from room temperature to 500° C. over a 3 hour period, holding at 500° C. for 2 hours, and cooling back to room temperature over a 3 hour period. Provided are monodisperse silica microspheres having an average diameter of 15 microns, containing two different average pore sizes.

FIG. 2 and FIG. 3 are scanning electron microscope (SEM) images of a polymer template microsphere and a porous silica microsphere prepared in a similar fashion.

Example 3 Porous Silica Microspheres Containing a Light Absorber

The product of Example 2 is physically mixed with an aqueous dispersion of carbon black or with a carbon black powder at varying weight levels. Provided are monodisperse porous silica microspheres containing carbon black at levels of 0.5 wt %, 1 wt %, 2 wt %, 3 wt %, 4 wt % and 5 wt %, based on the total weight of the microspheres.

Example 4 Drying Methods

Examples 1-3 are repeated, wherein the drying step employs microwave irradiation, drying under vacuum and/or drying in the presence of a desiccant.

Example 5 Preparation of Porous Silica Microspheres Via Spray-Drying

A styrene/acrylic acid copolymer is prepared as follows: 230 mL deionized (DI) water is added to a 3-neck reaction flask equipped with a thermometer, condenser, magnetic stirring and nitrogen atmosphere. The water is heated to 80° C. and 10 g of styrene are added with stirring, followed by 100 mg acrylic acid dissolved in 10 mL DI water via syringe. 100 mg of ammonium persulfate is dissolved in 10 mL DI water and added to the stirred mixture via syringe. The reaction mixture is stirred for 24 hours at 80° C. The polymer colloid dispersion is allowed to cool to room temperature and is purified via centrifugation, producing polystyrene nanospheres having an average particle size of 250 nm.

Similarly, a styrene/acrylic acid copolymer is prepared to produce polystyrene nanospheres having an average particle size of 350 nm.

The first aqueous polystyrene colloid dispersion (250 nm) is mixed with the second aqueous polystyrene colloid dispersion (350 nm) in a wt/wt ratio of 7/3 and the mixture is diluted to 1 wt % with deionized water and 1 wt % silica nanoparticles are added to the mixture which is sonicated to prevent particle agglomeration. The aqueous dispersion is spray-dried to provide polymer template microspheres comprising polydisperse polymer nanospheres and silica. The microspheres are calcined by heating from room temperature to 500° C. over a 3 hour period, holding at 500° C. for 2 hours, and cooling back to room temperature over a 3 hour period. Provided are porous silica microspheres.

Example 6 Visible Color in a Bulk Sample

In these bulk color examples, 0.5 milligrams of porous microspheres are evenly placed in a 10 mL clear glass vial having a 6 cm² bottom surface. The color is observed with the human eye.

A sample of porous silica microspheres is prepared in a similar fashion to Example 2, where with polystyrene nanospheres having average particle sizes of 420 nm and 460 nm in a wt/wt ratio of 7:3. A sample exhibits a red color.

A sample of porous silica microspheres is prepared according to the process of Example 5, where the polystyrene nanospheres have average particle sizes of 360 nm and 420 nm in a wt/wt ratio of 4:1 and the wt/wt ratio of polymer to silica is 4:1. Porous microspheres are provided having a porosity of 0.55 and which exhibit a distinct green color. Samples are also prepared where the wt/wt ratio of polymer to silica is 2:1, providing porous microspheres having a porosity of 0.45 and which exhibit a distinct orange color.

Example 7 Zinc Oxide Porous Microspheres

A sample of porous zinc oxide microspheres is prepared according to the process of Example 5, where the polystyrene nanospheres have average particle sizes of 250 nm and 320 nm in a wt/wt ratio of 1:1, and where the wt/wt ratio of polymer to zinc oxide is 1:2.

Example 8 Silica/Titania Porous Microspheres

A sample of porous microspheres containing silica and titania is prepared according to the process of Example 2, where the polystyrene nanospheres have average particle sizes of 350 nm and 460 nm in a wt/wt ratio of 1:4 and wherein the wt/wt ratio of polymer to total metal oxide is 3:1. The wt/wt ratio of silica to titania is 9:1. 

The invention claimed is:
 1. Porous microspheres comprising a metal oxide, wherein the porous microspheres have: an average diameter of from about 0.5 μm to about 100 μm; and an average porosity of from about 0.10 to about 0.80; wherein the porous microspheres have two or more populations of pores each having an average pore diameter, wherein each population has a different average pore diameter, wherein the average pore diameters of each population are independently selected from about 50 nm to about 999 nm, and wherein each porous metal oxide microsphere comprises the two or more populations of pores formed throughout a continuous solid structure of the metal oxide.
 2. The porous microspheres of claim 1, wherein the two or more populations of pores consist of a two populations of pores characterized by a bimodal size distribution.
 3. The porous microspheres of claim 1, wherein the porous microspheres have an average diameter of from about 0.5 μm to about 50 μm.
 4. The porous microspheres of claim 1, wherein the average pore diameters of each population are independently selected from about 50 nm to about 800 nm.
 5. The porous microspheres of claim 1, wherein the porous microspheres have an average porosity of from about 0.45 to about 0.65.
 6. The porous microspheres of claim 1, wherein the porous microspheres have: an average diameter of from about 4.5 μm to about 9.9 μm; an average porosity of from about 0.45 to about 0.65; and average pore diameters of each population independently selected from about 220 nm to about 300 nm.
 7. The porous microspheres of claim 1, comprising from about 60.0 wt % to about 99.9 wt % of the metal oxide, based on the total weight of the porous microspheres.
 8. The porous microspheres of claim 1, wherein the metal oxide is selected from the group consisting of silica, titania, alumina, zirconia, ceria, iron oxides, zinc oxide, indium oxide, tin oxide, chromium oxide, and combinations thereof.
 9. The porous microspheres of claim 1, wherein the porous microspheres are monodisperse.
 10. A composition comprising: a substrate; and porous microspheres comprising a metal oxide, wherein the porous microspheres have: an average diameter of from about 0.5 μm to about 100 μm and an average porosity of from about 0.10 to about 0.80; wherein the porous microspheres have two or more populations of pores each having an average pore diameter, wherein each population has a different average pore diameter, wherein the average pore diameters of each population are independently selected from about 50 nm to about 999 nm, and wherein each porous metal oxide microsphere comprises the two or more populations of pores formed throughout a continuous solid structure of the metal oxide.
 11. The composition of claim 10, wherein the composition is an aqueous formulation, an oil-based formulation, an ink, a coating formulation, a food, a plastic, a cosmetic formulation, or a material for a medical application or a security application.
 12. The composition of claim 10, further comprising one or more light absorbers.
 13. The composition of claim 12, wherein the one or more light absorbers are present from about 0.1 wt % to about 40.0 wt %, based on the total weight of the porous microspheres.
 14. The composition of claim 10, wherein a bulk sample of the composition exhibits angle-dependent color observable by a human eye.
 15. The composition of claim 10, wherein a bulk sample of the composition exhibits angle-independent color observable by a human eye.
 16. The composition of claim 10, wherein the metal oxide is selected from the group consisting of silica, titania, alumina, ceria, iron oxide, zinc oxide, and combinations thereof.
 17. The composition of claim 10, wherein the porous microspheres are monodisperse.
 18. The composition of claim 10, wherein the porous microspheres have an average diameter of from about 0.5 μm to about 50 μm.
 19. The composition of claim 10, wherein the average pore diameters of each population are independently selected from about 50 nm to about 500 nm.
 20. The composition of claim 10, wherein the porous microspheres have an average porosity of from about 0.45 to about 0.65. 